1. Field of the Invention
The present invention relates to a toner for use in electrophotography. In addition, the present invention also relates to a method of preparing the toner.
2. Discussion of the Background
In electrophotography, electrostatic recording, electrostatic printing, etc., a developer firstly adheres to an electrostatic latent image formed on an image bearing member (such as a photoreceptor) in a developing process. Secondly, the developer is transferred from the image bearing member onto a transfer medium (such as a transfer paper) in a transfer process. Finally, the developer is fixed on the transfer medium in a fixing process. As a developer, a two-component developer consisting of a carrier and a toner, and a one-component developer consisting essentially of a toner (e.g., a magnetic toner, a non-magnetic toner) are known.
Conventionally, a pulverized toner, in which a toner binder (such as a styrene resin and a polyester resin) and toner components (such as a colorant) are melt-kneaded and pulverized, is used for electrophotography, electrostatic recording, electrostatic printing, etc.
(Problems in Chargeability)
In the two-component developer, a toner is friction-charged by contacting a carrier. In the one-component developer, a toner is friction-charged by contacting a supplying roller which supplies the toner to a developing sleeve or a toner layer thickness controlling blade which makes a toner layer formed on the developing sleeve uniform. In order to faithfully reproduce an electrostatic latent image formed on a photoreceptor, the toner is required to have good chargeability. Various attempts to use charge controlling agents and to combine them to a toner have been made to improve the chargeability.
Charge controlling agents are very expensive in many cases. Since the charge controlling agent functions at the surface of a toner, attempts to arrange a small amount of a charge controlling agent on the surface of a toner have been made. Published unexamined Japanese Patent Applications Nos. (hereinafter referred to as JP-A) 63-104064, 05-119513, 09-127720, and 11-327199 have disclosed toners, on the surface of which a charge controlling agent is adhered, to improve chargeability thereof. However, the charge controlling agent easily released from the toner in the above cases, and therefore chargeability of the resultant toner deteriorates. The methods of adhering the charge controlling agent are also insufficient to impart good chargeability (such as initial charging rate) to the resultant toner.
JP-A 63-244056 discloses a method of preparing a toner in which a charge controlling agent is adhered and fixed onto the surface of the toner upon application of an impulse force generated from a gap formed between a rotor (i.e., a blade rotating at a high speed) and projections fixed on an inner wall of a stator (i.e., a casing). The inner wall is not smooth because the projections are formed thereon, and therefore a high-speed airflow tends to be turbulent. As a result, toner particles tends to be excessively pulverized, a part of the surfaces of toner particles tend to melt, and the charge controlling agent tends to be buried in the surface of toner particles, resulting in uneven treatment of the surface of toner particles. It is considered that the above phenomena are caused by the differences in the energy levels among individual toner particles.
(Problems in Fixability)
When a toner is fixed, contact heat fixing methods such as a heat roller fixing method and a belt fixing method (disclosed in JP 3195362 and JP-A 2002-116574, for example) are typically used.
In contact heat fixing methods, a toner image formed on a transfer paper sheet is contacted with the surface of a heat member of a contact heat fixing device. Therefore, the contact heat fixing methods tend to cause an offset problem in that part of a fused toner image is adhered and transferred to the surface of the heat member, and then the part of the toner image is re-transferred to an undesired portion of the sheet itself or the following sheet of the transfer paper.
In addition, the contact heat fixing methods tend to cause a paper winding problem such that a transfer paper sheet having a melted toner image thereon is wound around fixing members (such as a heat roller and a fixing belt) contacting the toner image due to adhesion of the toner image to the fixing members.
When the temperature of the heat member is too low, the toner cannot sufficiently melt and cannot be well fixed on the paper. In terms of energy saving and downsizing of apparatuses, a need exists for a toner which minimizes hot offset (this property is hereinafter referred to as hot offset resistance) and which can be fixed at low temperatures (this property is hereinafter referred to as low-temperature fixability). The toner is also required to have a property such that the toner does not cause a blocking problem even when the toner is stored at the temperature inside an apparatus (this property is hereinafter referred to as thermostable preservability). In particular, full-color copiers and printers are required to produce images having glossiness and color-mixing property, and therefore polyester resins are widely used as a full-color toner binder because of having low-melt viscosity. Since such a toner easily causes hot offset, a silicone oil is typically applied to a heat member in the full-color copiers and printers. In this case, the apparatus needs an oil tank and an oil applicator, and therefore the apparatus must be larger and complicated. In addition, the heat member is easily damaged, and therefore maintenance has to be constantly performed. There is another problem such that the oil applied to the heat member tends to adhere to copier papers and overhead projection (OHP) films, resulting in deterioration of the color tone of the produced images.
In attempting to solve these problems, a technique in which a wax is added to a toner is proposed and widely used to prevent the toner from adhering to the heat roller without applying an oil thereto. Releasability of the toner greatly depends upon dispersing conditions of the wax in the toner. When the wax is compatible with the binder resin used, the toner has no releasability. When the wax is incompatible with the binder resin and forms domains thereof in the toner, the toner has releasability. In this case, the wax domains tend to expose at the surface of the toner in the manufacturing process. Thereby, fluidity of the toner deteriorates, resulting in deterioration of transferability thereof. In addition, the wax tends to contaminate image forming members such as a photoreceptor, a carrier, a developing roller, and a controlling member. In a pulverized toner, the wax tends to exist at pulverized sections, i.e., the wax tends to exist at the surface of the toner particles.
(Problems in Particle Diameter and Shape)
In order to produce high definition and high quality images, toners are improved to have a smaller particle diameter and a narrower particle diameter distribution. Since conventional pulverized toner particles have irregular shapes, the toner particles tend to be excessively pulverized when mixed with a carrier in a developing device (when used for a two-component developer), or when contacting a developing roller, a toner supplying roller, a toner layer thickness controlling blade, a friction-charging blade, etc. under stress (when used for a one-component developer). As a result, the resultant image quality deteriorates because ultra-fine particles are produced and a fluidizer is buried in the surfaces of the toner particles. Since such an irregular-shaped toner has poor fluidity, there is a problem that the toner needs a large amount of a fluidizer. Another problem is that a toner bottle must be larger because such an irregular-shaped toner cannot effectively fill up the toner bottle, resulting in disturbing downsizing of the apparatus.
A full-color transfer process in which a full-color toner image is transferred from a photoreceptor to a transfer medium or a paper is complicated. On the other hand, a pulverized toner has poor transferability due to its shape. When the pulverized toner is used for the full-color transfer process, the transferred image may have image defects and a large amount of the toner is consumed so as to compensate the image defects.
Therefore, there are demands for improving transferability of the toner to produce high quality images by reducing image defect and to reduce the running cost by reducing the amount of the toner consumed. If the toner has good transferability, the toner particles tend not to remain on the photoreceptor or the transfer medium, and therefore the apparatus does not need a cleaning unit. As a result, the apparatus can be downsized and the cost thereof can be reduced. In addition, waste toner particles are not produced. In attempting to overcome the above drawbacks of the irregular-shaped toner, various methods of preparing a spherical toner are proposed.
For example, in attempting to improve both low-temperature fixability and hot offset resistance of a toner, a technique in which a release agent (e.g., a polyolefin wax) having a low melting point is added to a toner is proposed.
JP-As 06-295093, 07-84401, and 09-258471 have disclosed toners including a wax having a specific endothermic peak measured by a differential scanning calorimeter (DSC). However, these toners do not sufficiently satisfy low-temperature fixability, hot offset resistance, and developability.
JP-As 05-341577, 06-123999, 06-230600, 06-295093, and 06-324514 have disclosed toners including a release agent such as a candelilla wax, a higher fatty acid wax, a higher alcohol wax, natural plant waxes (a camauba wax, a rice wax), and a montan ester wax. However, these toners do not sufficiently satisfy low-temperature fixability, hot offset resistance, developability (chargeability), and durability. In general, when a release agent having a low-melting point is added to a toner, fluidity of the toner deteriorates, and therefore developability, transferability, chargeability, durability, and preservability thereof also deteriorate.
JP-As 11-258934, 11-258935, 04-299357, 04-337737, 06-208244, and 07-281478 have disclosed toners including two or more release agents so as to broaden the fixable temperature range (in which hot offset does not occur) thereof. However, these toners have a problem in dispersibility of the wax in the toner.
JP-A 08-166686 discloses a toner including a polyester resin and two offset inhibitors, each of which has an acid value and a different melting point. However, this toner has insufficient developability.
JP-As 08-328293 and 10-161335 have disclosed toners including wax particles having a specific particle diameter. However, the existential condition and location of the wax particles are undefined, and therefore the toner has insufficient separativeness when fixed.
JP-A 2001-305782 discloses a toner, on the surface of which spherical wax particles are fixed. When the wax particles are present on the surface of the toner, fluidity thereof deteriorates, and therefore developability, transferability, chargeability, durability, and preservability also deteriorate.
JP-A 2002-6541 discloses a toner in which wax particles are locally present near the surface of the toner particle, which is prepared by dispersing a heat-melted mixture including the wax, a condensation resin, and a colorant, in a hot water, followed by drying. When the melted mixture of the resin, the colorant, and the wax is dispersed in the hot water, the wax particles migrate to the surface region of the toner particle, and then the mixture is subjected to cooling so as to be solidified. In this case, the outermost region of the toner includes a larger amount of the wax than the inner adjacent region thereof. When the inner adjacent region includes a larger amount of the wax than the outermost region, the wax forms a continuous phase thereof but does not form domains thereof (as illustrated in FIG. 3 of JP-A 2002-6541). When the wax does not sufficiently migrate when melted, the wax present at the center of the toner increases (as illustrated in FIG. 2 of JP-A 2002-6541). It is described therein that the above-mentioned toner in which wax particles are locally present near the surface of the toner particle has good thermal stability when stored. However, the toner does not have sufficient resistances to mechanical and thermal stress applied thereto. The toner described in one or more Examples thereof includes a wax in an amount less than 1% by weight based on total weight of the toner, and therefore the wax tends not to adhere to a fixing member. However, the amount of the wax is too small to exert good separativeness when the toner is fixed, resulting in poor hot offset resistance of the toner.
JP-A 2002-91060 discloses a toner including a wax and having a core-shell structure. The shell is formed of a binder resin having a high glass transition temperature (Tg), and the core is formed of a binder resin having a low Tg and includes the wax therein. Since the wax is located in the center of the core covered with the shell formed of the binder resin having a high Tg, the wax cannot sufficiently exude to the surface of the toner, resulting in poor separativeness of the toner. When the amount of the wax is too large, the wax exuded from the toner is projected when an image is formed on an overhead projection (OHP) sheet, resulting in deterioration of image quality of the projected image.
JP-As 2004-145243 and 2004-318043 have disclosed toners in which specific amount of a wax is present near the surface of the toner. It is described therein that such a toner has good thermostable preservability and chargeability, while having good separativeness between a fixing member. No mention is made of whether the surface includes the wax. In either case, the wax is present very near the surface of the toner, and therefore the wax tends to release therefrom when a load is applied thereto in the image forming process for a long period of the time, resulting in contamination of a photoreceptor, a carrier, a developing roller, a controlling member, etc.